In a cellular radio network, a certain service area is covered by several radio base stations, which are connected to a mobile switching center to serve calls to and from mobile users within the service area. In the new UMTS standard, such a radio base station is also termed a “node B”. A radio base station contains a baseband unit and at least one antenna unit. In order to increase radio coverage and capacity, modern base stations use several sector antennas.
In order to increase flexibility of the base stations, it is desirable to allow the antennas to be located remote from the baseband unit. This has led to the development of active antenna systems which are also termed radio frequency heads (RF heads). Typically, one RF head contains one sector antenna, but systems are also known which have RF heads with more than only one sector antennas. RF heads must, however, be located quite close (distance up to 30 m) to the baseband unit to limit the losses occurring on the RF cables connecting the two. Moreover, the use of longer interconnections would lead to problems with different propagation delays between baseband unit and the several RF heads.
The recently published Common Public Radio Interface (CPRI) Specification V1.0, 30.09.2003, describes an internal interface of radio base stations between the baseband unit, which is termed Radio Equipment Control (REC), and the RF head, which is termed the Radio Equipment (RE). The document defines an optical interface for base stations with remote radio equipment and a delay calibration procedure, which estimates the path delay between REC and RE by measuring the round trip delay and divide it by 2. The specification assumes, however, bi-directional point-to-point connections between the REC and each RE, which may be cumbersome and expensive to implement for a base station with multiple RF heads.
U.S. Pat. No. 5,809,395 describes a radio base station where the antennas are associated with active antenna systems and the active antenna systems are connected to the base stations utilizing broadband transmission by means of a fixed bi-directional signal distribution network.
It is an object of the present invention to provide a radio base station with multiple RF heads which are connected to a baseband unit in a simple and efficient way.